The invention concerns a method of producing boron (B)-doped monocrystalline silicon carbide.
In addition to aluminum, boron is the most important dopant for p-type doping of monocrystalline SiC semiconductor material.
In Applied Physics Letters, vol. 42, no. 5, Mar. 1, 1983, pages 460-462, a process is disclosed for producing a boron-doped layer of monocrystalline silicon carbide (SiC) of the 3C polytype (.beta.-SiC) by chemical vapor deposition (CVD) on a monocrystalline silicon substrate at a temperature of 1400.degree. C. In this known process, hydrogen (H.sub.2) is used as the carrier gas, silane (SiH.sub.4) is used as the precursor for supplying silicon (Si) with a molar amount of 0.04% in the H.sub.2 carrier gas, and propane (C.sub.3 H.sub.8) with a molar amount of 0.02% in H.sub.2 is used as the precursor to supply carbon (C). The doping gas diborane (B.sub.2 H.sub.6) is added to the gas mixture of the carrier gas and the two precursors in the amount of 100 ppm in H.sub.2 for doping. The deposited .beta.-SiC layer has a charge carrier concentration of holes (p-type conduction) of 5.6.multidot.10.sup.14 to 1.6.multidot.10.sup.15 cm.sup.-3.
In another process disclosed in Journal of the Electrochemical Society, vol. 133, no. 11, November 1986, pages 2350-2357, a boron-doped .beta.-SiC layer is produced by CVD epitaxy at a temperature of 1360.degree. C. with silane (SiH.sub.4) and ethene (C.sub.2 H.sub.4) as precursors, hydrogen (H.sub.2) as the carrier gas and diborane (B.sub.2 H.sub.6) as the doping gas. With an SiC layer produced by this known method, only a small portion (0.2%) of the boron atoms introduced into the SiC are electrically active. To achieve a high charge carrier concentration of the p-type conduction, the atomic concentration of boron in SiC must therefore be so high that the surface quality of the growing SiC layer is greatly impaired.
U.S. Pat. No. 4,923,716 discloses another CVD process for producing boron-doped .beta.-SiC with diborane as the doping gas.
Furthermore, sublimation processes are also known for producing monocrystalline SiC where an SiC bulk crystal of sublimed SiC is grown in the vapor phase (mainly Si, Si.sub.2 C, SiC.sub.2) on the wall of a vessel (Lely process) or on a seed crystal (modified Lely process).
Furthermore, plasma-assisted CVD process are also known for deposition of a boron-doped amorphous compound of silicon and carbon, a-Si.sub.1-x C.sub.x :H, with hydrogen inclusions on a substrate. For doping the a-Si.sub.1-x C.sub.x :H layer with boron, an organic boron compound with an unsaturated hydrocarbon residue or boron trimethyl or boron triethyl is added to a precursor containing hydrogen gas and silane. The deposition temperatures on the substrate are between 150.degree. C. and a maximum of 300.degree. C. The optical energy gap of the a-Si.sub.1-x C.sub.x :H layer is adjusted through the boron doping. Such a-Si.sub.1-x C.sub.x :H layers are used as p-type layers in a p-i-n solar cell based on amorphous silicon. The amount of silicon in these known a-Si.sub.1-x C.sub.x :H layers is much greater than the amount of carbon (European Patent A 573,033 or Patent Abstracts of Japan, C-711, Apr. 19, 1990, vol. 14, no. 192 or Physica B, no. 170 (1991) pp. 574-576 or Journal of Non-Crystalline Solids, no. 137+138 (1991) pp. 701-704 or Materials Research Society Symposium Proceedinqs, vol. 118 (1988) pp. 557-559 or Philosophical Magazine B, vol. 64, no. 1 (1991) pp. 101-111). Monocrystalline SiC layers cannot be produced by these known methods.